This invention relates to surgery in which an implant is fixed in a bone by the use of bone cement, e.g. polymethylmethacrylate bone cement.
It has been well established by laboratory experiments and clinical experience that the fixation of implants such as the components of replacement joint protheses by selfcuring polymethylmethacrylate bone cement is enhanced if the bone cement is put under pressure before the implant is inserted. The pressurisation has two principal effects: (a) it improves the quality and strength of the bone cement material, and (b) it improves the strength of the interlock between the cement and the bone into which the cement is placed, since the cement is thus encouraged to penetrate into the interstices of the bone structure. Both these effects are desirable.
A number of devices are available which attempt to carry out such cement pressurisation. A typical device is that developed by the present inventors and shown in U.K. Pat. No. 1,430,083. This specification describes an inflatable seal, and its method of use is as follows. Firstly, the cavity in the bone into which the cement is to be placed is formed by the surgeon. The cavity should be open at one part only, and the medullary canal, for example, will need to be sealed at its distal end by a suitable plug before insertion of the cement. Such a plug is the subject of U.K. Patent Specification No. 2,054,383. Once the cavity in the bone is completed, bone cement is introduced into the cavity either by hand or by using a cement syringe or gun similar to that described in U.K. Patent Specification No. 1,603,102. After withdrawal of the syringe or gun, pressurisation may now take place by forming a seal over the bone cement, completely enclosing the opening of the bone cavity, using the inflatable pressuriser device of U.K. Pat. No. 1,430,083. The pressuriser is then forced manually down onto the surface of the cement, causing the total volume of the closed cavity to be descreased. The cement, being virtually incompressible, is put under pressure and forced to flow into the pores of the bone itself. (These bony pores are often the trabecula bone.) Thus, this procedure involves two steps: firstly the bone cavity is filled with cement, and secondly a pressuriser device is applied to decrease the total volume of the closed cavity.